DESCRIPTION (Applicant's Abstract): Friedreich's Ataxia (FRDA) is the most common hereditary ataxia and is associated with mutations in the nuclear gene X25, which encodes the protein frataxin. The overall goals of the proposed research are 1) to determine the function of frataxin by elucidating the function of its homolog, FRH, in S. cerevisiae, and 2) to understand the role of frataxin in the cellular pathophysiology of Friedreich's Ataxia by studying disease-associated missense mutations in S. cerevisiae. Preliminary studies demonstrate that yeast lacking the FRH gene are respiration deficient due to a complete absence of mitochondrial DNA. The protein encoded by FRH localizes to mitochondria, and the C-terminal homology domain of the human X25 functions normally in yeast when substituted for the corresponding domain of FRH. When a missense mutation corresponding to a missense mutation in FRDA patients is introduced into FRH or into the C-terminal homology domain of an X25 construct in yeast, respiratory function is significantly reduced. Thus yeast carrying disease-associated missense mutations in FRH or LX25 can serve as a model system for understanding the cellular pathophysiology underlying FRDA. The Specific Aims are 1) to determine the cause of the respiratory dysfunction in yeast carrying disease-associated missense mutations of FRH or X25 by analyzing mitochondrial DNA, ultrastructure, and membrane composition, and 2) to further elucidate the function of FRH in yeast by isolating and studying FRH-interacting proteins and extragenic suppressors of FRH mutations. Methods include Southern and Northern analyses, electron microscopy, and membrane fatty acid analysis of yeast carrying the missense mutants, as well as dihybrid cloning and extragenic suppressor analysis. The hypotheses to be tested include that a defect in mitochondrial DNA replication or repair, or a mitochondrial membrane defect, is the primary cause of the respiratory dysfunction. The results of these studies will further the understanding of the pathophysiology of Friedreich's Ataxia and are therefore relevant to the development of novel methods of prevention and treatment.